Polyvalent bifunctional catalyst and the process of realization of such a catalyst

ABSTRACT

The objective of the present invention is a polyvalent bifunctional catalyst and the process of its realization. A catalyst characterized by the fact that it contains, over a TiO 2  support, an oxide or a mixture of metallic oxides of MC 2  type obtained by reduction of the corresponding MC 3  oxides, the metal(s) forming the MO 2  oxides are chosen from the group formed by W and Mo.

The present invention concerns the field of catalysis, particularly thefield of heterogeneous catalysis.

Its object is a catalysis system based on metallic oxides of the MO₂type.

Its object is also the process of obtaining catalytic systems as well asthe application of those systems in the chemical industry, especially inpetro-chemistry, for the reactions of isomerization, dehydrogenation,hydrogenation and/or in hydrogenolysis of saturated and/or unsaturatedorganic compounds.

In the chemical industry, most reactions are accelerated by catalystswhose function is to allow the progress of those reactions underconditions of temperature and pressure which are economicallyprofitable.

We already know four major types of catalysts in the field ofheterogeneous catalysis or (contact catalysis):

The pure metals(4e, 5e, and the 6e periods of transition metals)

The oxides of transition metals or of some heavy elements showingseveral stable oxidation states.

The solid oxides of metals from the two first columns IA and IIA of theperiodic table.

The light metals and non-metals from columns IB, IVB, VB (acid oxides)

Within those different categories, we can distinguish two sub-categoriesaccording to their way of preparing this system: Bulk catalyst andsupported catalyst, i.e. whose active phase is deposited on a support.

In the chemical industry in general, acid oxides (alumina, silica, ofenmixed, zeolites) catalyse mainly the hydration, isomerization,alkylation and cracking of organic molecules.

Certain oxides can catalyse at the same time redox reactions and acido-basic reactions: these are called bifunctional catalysts (reformation offuels, synthesis of butadiene).

In the petrochemical industry in particular, the use of catalysts in theisomerization process, allows the obtaining of hydrocarbons with highoctane number which can be used directly.

However, the catalysts used nowadays for the types of reactionsmentioned above, still present a lot of inconveniences, some of whichare important.

Indeed, a big part of the known catalysts contain noble metals such asplatinum, paladium or iridium. The content, even if.very small, of suchmetals in the known catalysts, as well as the difficulties in recyclingthem, explain the very high prices of such systems.

Furthermore, research trying to replace those noble metals by cheapermetals in order to obtain new efficient catalytic compounds has notbrought any really satisfying solutions up to now.

In particular, supported metallic catalysts, well-known for theiractivity in terms of hydrogenolysis and isomenzation, have been thesubject of studies on the substitution of noble metals by oxides oftransition metals in particular.

However, the physico-chemical performances in terms of conversion,selectivity, life-time and recycling capability of the catalystsproposed at the end of these studies, are not always up to theindustrial expectations, which is all the more prejudicial, as a goodnumber of those new catalysts are often usable for a limited number ofcompounds and for specific reactions. Moreover, the development of acatalyst with the exact objective is generally uncertain, long andexpensive.

As far as the bifunctional catalysts are concerned, one has to note thatthe acid character is brought by the support, which nowadays is usuallyan acid or chlorated alumina, eventually a zeolithe, whereas themetallic character is brought by a deposited metal. The necessarypresence of two active substances leads to problems too, which are allthe more important as these active substances are different, problemssuch as manufacturing, high costs, incompatibility between the materialsand their treatment.

The extensive hydrogenolysis properties of tungsten and molybedenumcarbides have been clearly demonstrated by A. Katrib et al. Cat. Lett.,38(1996)95, and it has been shown that the presence of oxygen leads tothe formation of oxycarbides WO_(x)C_(y) type compounds which providethe isomerization catalytic properties to these new systems.

An identification, in particular by X-ray photoelectron spectroscopy ofthese new systems enabled to identify the active species as WO₂ and MoO₂with isomerization properties (A. Katrib et al. J. Electron. Spectro.Relat. Phenomenon. 76(1995)195, and J. Chim. Phys. 94(1997)1923). On theother hand, the existence of the oxycarbide species WO_(x)C_(y) has beenexcluded. Also, it has been shown that the W or Mo pure metals havehydrogenolysis properties, whereas the WO₃ and MoO3 trioxides arecatalytically inactive concerning saturated hydrocarbos.

Certain research on these trioxides deposited on alumina(W. Grünert etal. J. Cat. 107(1987)522) have shown that the support stabilises theoxide WO₃ in terms of a strong metal-support interaction. It istherefore difficult to form WO₂, which is responsible for the catalyticactivity on such a support.

In the isomeriztion catalysts, we can refer to the works of Martin (C.Martin et al. Cat. Lett. 49(1997)235) which describes the overalltechnical experiments allowing to characterize this type of catalysts.Moreover. Vermaire and Van Berge (J. Cat. 116(1989)309) have directedtheir work on the preparation of isomedzation catalysts, emphasizingamong others, the influence of the pH, which is less important in thecase of TiO₂ than on Al₂O₃. They also proposed a mehanism allowing tointerprete the stoichiometric ratio 1:1 during the adsorption of the WO₃on the sites Ti—O—Ti type, and they have shown the importance of amonolayer of WO₃ deposited on TiO₂. According to them, this onecorresponds to the maximum amount of tungsten which can accurnaate onTiO₂ when it is placed for impregnation in a solution of pH=2. However,Rondon, Howalla and Herculs have established that this maximum quantitydepends on the pH of the impregnation solution (Surf. Interface Anal.26(1998)329).

The works of Yamaguchi, Tanaka and Tanabe (J. Cat. 65(1980) 442) haveshown that the activity of catalysts based on tungsten studied in theirwork, reached a limit value for an initial content of WO₃ of 8% (molar).This quantity corresponds in fact to the triple of the formerlyidentified monolayer. They have also correlated activity to the aciditywhich occurs during the mixing of the two oxides TiO₂ and WO₃. Thisacidity of Lewis type is interpreted by the accumulation of positiveelectrical charges on the tungsten according to the theory establishedby Tanabe et al. (Bull. Chem. soc. Japan 47(1974)1064). It is thenpossible, in the presence of water, to obtain the Brönsted acidity.

Finally, Hino and Arata (Bull. Chem. soc. Japan 67(1994)1472), haveprepared solid superacid, by impregnating titanium hydroxide with WO₃.It is not really a question of deposition of WO₃ on a suport, since thesupport (TiO₂) is obtained by calcining the corresponding hydroxideafter impregnation of the tungsten species. The application of thesecatalysts for the reactions of isomerization has not been considered.

The problem to be solved by the present invention consists therefore insupplying a bifunctional catalyst, which is cheap and stable in functionof time, polyvalent and performing.

To this purpose, its object is a polyvalent bifunctional catalyst,characterized by the presence of MO₂ type phase, supported on TiO₂. ThisMO₂ phase is obtained by the reduction of the corresponding MO₃oxide(s). The metal(s) forming the MO₂ oxides are preferrably chosen inthe group formed by W and Mo.

The invention will be better understood due to the following descriptionwhich relates to the preferred preparations, which are presented as anon-limited examples, and are explained with reference to the figuresattached as enclosures:

FIG. 1 represents in a schematic way, an example of installation of areactor plant oriented toward the preparation and the study of thecatalyst there after.

FIG. 2 represents a diagram showing a comparaison between the activitiesof the calcined and the non-caiined mechanical mixture of the catalystsC1 and C2 in function of the time of reduction using 2-methylpentanereactant.

FIG. 3 represents a diagram showing the activity level and thestabilisation of the catalyst C4 using 2-methylpentane reactant.

According to the invention, the bifuinctional polyvalent catalyst isformed by an oxyde or a mixture of oxides of MO₂ type, deposited on asupport TiO₂. The oxides MO₂ being obtained by the reduction of thecorresponding MO₃ oxides.

The metal(s) forming the MO₂ oxides are preferrably chosen in the groupformed by W or Mo so that the metallic oxide obtained by reduction onits support is the oxide of tungsten WO₂ or the oxide of molybdenumMoO₂. Of course, mixtures between the oxides of tungsten and molybdenumcited before are possible.

It has been found in a surprising and unexpected way, that a reductionin situ of the initial state of the commercial oxides, MO₃ and MO₂, isalways necessary in order to observe a catalytic activity. Indeed, evenby starting with the commercial dioxide MO₂ or the metal powder M, thereis always a certain number of layers of the corresponding trioxide MO₃present on the surface, which it is necessary to reduce to MO₂ in orderto observe a catalytic activity.

As a non-limitative example, the isomerizing activity of WO₂ can beinterpreted by the bifunctional character of this species, in similarway to the supported metallic catalysts.

Indeed, the dioxide behaves like platinum supported on acidic alumina,the W⁴⁺ (WO₂) species having two free electrons, in contrary to W⁶⁺.

The two free pre-cited electrons lead, from one side, to the formationof σ bonds between the aligned tungsten atoms in WO₂. On the other hand,it lead to the formation of π bonds between two tungsten atoms in twoneighbouring sites in WO₂. The delocalisation of the π electrons acquirethe metallic properties to the oxide, which enable the fonctions ofhydrogenation and/or dehydrogenation and the dissociation of hydrogen(hydrogen molecule H₂).

The pre-cited electrons can be observed as a density of state at theFermi-level in the X-ray and Ultraviolet photoelectron spectroscopy.Therefore it is possible to distinguish between the presence of two W—Wbond lengths in WO₂, depending on the presence or not of the π bond. Theprotonation of the surface oxygen atoms leads to the formation ofBronsted acidic sites within the material. These acidic sites enable theisomerization functions, i.e. the displacement of the hydrocarbonchains.

The choice of TiO₂ as a support, is essential for the present invention,and it brings numerous important advantages to the catalytic compoundsof the present invention.

Indeed, titanium dioxide is present in two cristalline varieties(A. D.Waddey, rev. pure. Appi. chem. 5(1955)165), called rutile and anatasehaving a tetragonal structure with slightly different latticeparameters.

The inventors have noticed that the TiO₂ rutile structure corresponds tothe WO₂ (deformed rutile structure) crystal structure. Therefore, thetwo oxides, TiO₂ and WO₂ have very close c stuctures with neighbouringgeometrical parameters.

According to the present invention, the metallic oxide(s) MO₃ reduced toMO₂ can be deposited on the TiO₂ support alone, or TiO₂ supported on asubstrate having larger surfine area than TiO₂.

We project for example, to deposit TiO₂ whose surface area in the ordeof 57 m²/g on the surface of metallic oxides, preferrably, SiO₂, Al₂O₃or a zeolite, where all these supports have surface areas of the orderof 180 m²/g.

The activation of the catalyst, by means of the reduction of the oxideMO₃ to MO₂ is an essential element of the present invention.

Indeed, initially, the metal, tungsten for example, is present in thestate of trioxide MO₃ (catalytically inactive) in all the preparedcatalysts. Those need therefore a reduction under hydrogen from MO₃ toMO₂ in order to activate the catalyst. This reduction process isrealised directly in the catalytic reactor FIG. 1.

In FIG. 1, one can see that the gases used are initially purified, bycirculating on a first trap 2, which reduces the eventual traces ofoxygen to water, which is trapped on a zeolithe trap. The outputs areregulated with the help of a volumic debitmeter 3 of tylan type and adebimeter of Brookes 4 for further control.

The gaseous flux circulates first of all through the compartiment ofreference of the catharometers 5, 5′. It goes then through a second trap6, refrigerated by liquid nitrogen for example, at the passage of whichan eventual injection of reacting hydrocarbons. The mixture which ismade this way passes over catalyst 7, placed in an oven 8. The passageon the catalyst is monitored by two catharometers 5, 5′, respectivelyplaced before and after the oven 8. At the outlet of oven 8, ahydrogenator containing heated platinum Adams 10, enables to hydrogenatethe unsaturated to saturated hydrocarbons which are analysed by gaschromatography. The physical parameters of the reaction ofreduction(temperature, and pressure) are measured by the thermocouple Tand the manometer P.

An advantageous realisation of the invention, is that the reductiontakes place under a gaseous flux containing at least hydrogen at atemperatures between 380 and 550° C., for at least 6 hours, and a gasflux which ranges between 0.010 l/min and 0.050 l/min, preferrably 0.030l/min. and containing a volume ranging betwen 90% and 100% hydrogen,preferrably 99% hydrogen. Under these conditions the catalytic activitystablises within 6 hours.

In another way of realisation, which is particularly practical todetermine the reduction temperature at which the catalytic activitystabilises, is for the reduction process to take place under a gaseousflux containing beside hydrogen, a gaseous hydrocarbon reactant which isgoing to be reacting on the catalyst. As non limitive examples, suchgaseous hydrocarbon compounds, we mention 2-methylpentane, n-heptane,4-methylpentene-1.

Also as an example, the gaseous hydrocarbon can be present in themixture under a partial pressure ranging between 666.6 Pa(5 torr) and1999.8 Pa(15 torr), preferrably 799.9 Pa(6 torr).

In an advantageous manner, the MO₃ oxide(s) are deposited in atomiclayers on a support material before being reduced into MO₂; thatreaction does not affect the number of atomic layers.

In order to obtain the best catalytic results in terms of selectivityand conversion, the number of atomic layers of MO₂ present on thesupport material ranges between 1 and 8 layers, preferrably 5. Thiscorresponds in practice to catalysts containing from 5.4% to 27% of massof MoO₃, which is the equivalent of 4.8% to 24% of mass of MoO₂ orbetween 6% to 30% of mass of WO₃ which corresonds to 5.7% to 28% of massof WO₂.

In accordance with a first variation of realisation, a catalystcorresponding to the invention is obtained by a simple mechanicalmixture of MO₃ and TiO₂.

So, according to a first process of obtention corresponding to thepresent invention we follow the following steps:

Preparing a mechanical nxxture of one or several MO₃ oxides with TiO₂ orTiO₂ deposited on a substrate presenting larger specific surface areathan TiO₂ alone. The mixture having a metal M content ranging between 5%and 25%, and preferrably 22%.

Crushing the obtained mixture in the first step.

Reducing, preferrably at 460° C., the deposited MO₃ oxide(s) to thecorresponding MO₂ oxide(s) by the introduction of a gaseous fluxcontaining at least hydrogen.

In this manner, the step of depositing the MO₃ oxide(s) is realised bymixing mechanically the crushed MO₃ oxide(s) with TiO₂ or TiO₂ depositedon a substrate presenting larger specific surface area than TiO₂ alonesuch as SiO₂.

We describe below in more details, an example of the process to obtain acatalyst according to the invention by mixing mechanically WO₃ withTiO₂.

The catalyst is obtained by crushing in a mortar a mixture of trioxideof tungsten which was calcined for 16 hours and dioxide of titanium(P25-Degussa).

Characterization of the support (TiO₂)

Porous volum=0.0005 l/g

Specific area=50 m²/g

A first catalyst C1 has been prepared whose content in tungstencorresponds to an atomic layer of WO₃ deposited on TiO₂. The quantity ofthe WO₃ necessary, is determined on the basis of the parameters of thelattice structure of WO₃ according to the values given in table 1 asfollows:

TABLE 1 Contents in oxydes and in metal for the catalyst C1. Finalcontent in Masse of Content in oxide metal (W/Ti) metal Masse [g] (%mass./% mol.) [g] (% mass.) TiO₂ 1 92.85%/97.5% 0.599 55.66% WO₃ 0.0777.15%/2.5% 0.061  5.67% Total 1.077 100%

A second catalyst C2 has been prepared by calcining catalyst C1 for 16hours at 500° C. This treatment of calcination is similar to thetreatment of calcination following the impregnation which will bedescribed later. The study of these catalysts has allowed to understandthe modifications which can occur during this step of calcination:sintering or diffusion of species in the solid state (Ceramic way).

In a second way of practical preferential realization of the catalystaccording to the invention, is a catalyst deposited on a support.

This process of obtaining a catalyst according to this way ofpreparation is characterized by the following steps:

Washing the raw support, drying and calcination

Crushing the obtained solid followed by sifting(separation of particlesizes).

Depositing the MO₃ oxide(s) on a support material made up of TiO₂ orTiO₂ deposited on a substrate presenting larger specific surface areathan TiO₂ alone by impregnation of the support material with a solutionof one or several salts of the metal M.

Calcination of the obtained product to form MO₃ oxyde(s).

Reduction, preferrably at 510° C. of the MO₃ oxide(s) by introducing agaseous flux containing at least hydrogen on the MO₃ oxide(s).

In a preferred way, only the particles with a diameter ranging from 80μm to 400 μm were conserved.

In accordance with the invention, the impregnation of the salt(s) of themetal M takes place between 2 to 4 hours, preferrably 3 hours, at atemperature ranging from 50° C. to 90° C., preferrably 70° C.

In accordance with other characteristics of the invention, one of thesalt(s) of tungsten or molybdenum could be used in order to obtain WO₃then WO₂ or MoO₃ then MoO₂, preferrably (NH₄)₁₀W₁₂O₄₁.5H₂O for W and(NH₄)₆Mo₇O₂₄.4H₂O.

As will be explained in more details below, the impregnation of thesupport can be done under a controlled pH or not. Otherwise, accordingto another characteristic of the invention, the impregnation of thesupport occurs under a constant pH ranging between 1 and 4, preferrablya pH equal to 2.

According to most preferred embodiment of the realisation which will beexplained in more details further on, it can be foreseen that the saltsolution is in excess with respect to the support volume which isevaporated after the impregnation in an oven at a temperature rangingbetween 80° C. and 120° C., preferrably at 100° C., for 10 to 14 hours,preferrably 12 hours.

For all of these processes of-obtention of the present invention, thenumber of atomic layers of MO₃ oxides deposited on the support rangesbetween 1 to 8, preferaably 5 layers, and as for all catalysts, themetal(s) forming the MO₂ oxide(s) in the processes according to thepresent invention are preferrably chosen in the group formed by W andMo.

In the known processes of dry impregnation one dissolves the precursorin a solution which occupies exactly the porous volume of the treatedsupport. Taking into account the small mass of TiO₂ used, and the smallporous volume of TiO₂ (0.5 cm³/g). A new process of a catalyst by usinga technique of impregnation by excess solution is established in thisinvention.

Such a process of impregnation by excess of a solution is described inthe work of Wang and Hall(J. Cat. 77(1982)232). It consists ineliminating by filtration the excess solution(ammonium paratungstate)which has been in contact with the support.

However, the inventors found that only an impregnation realized with apH which is sufficiently acid, allows to observe a significativecatalytic activity concerning the catalytic products studied in thiswork.

We are going to describe in more details an example of the process ofpreparation of a tungsten catalyst supported by wet impregnation at anon-controlled pH.

In this process, the solution containing the precursor occupies a volumewhich is much bigger than the porous volume alone. The quantities ofammonium paratungstate which are necessary for the deposit one or fivelayers of WO₃, have been dissolved in distilled water; the solutionsobtained that way, having been in contact with TiO₂. There are twopossibilities then:

For the catalyst C3 on which we wished to deposit an atomic layer, theexcess of solution has been eliminated by filtration. The catalyst hasbeen dried up in an oven (110° C.) then calcined at 500° C.

For the second catalyst C4 on which to deposit the equivalent of 5layers, the solution of impregnation has been eliminated by evaporation.This corresponds to the preferred variation of the impregnation processderived from the one proposed by ipatieff et al. The catalyst has beenthen treated like the precedent one (drying and calcination). Thequantities used as an example in catalyst C4 are given in table 2.

TABLE 2 The expected contents in oxydes and metals present in catalystC4. Mass Content Masses (oxides) [g] (oxides) (metal) [g] content(metal) TiO₂ 1 72.2% 0.599 43.24% WO₃ 0.385 27.8% 0.305 22.06% Total1.385  100%

We noted for the catalyst C3 that the theoretical contents of tungstenhave only partially been reached: The difference between the theoreticaland experimental metal values allows to take into consideration thelosses due to the use of this method in the case of the followingpreparation.

Another technique of impregnation by excess of solution at a controlledpH, is derived from the works of Rondon et al. as well as Wang et al.

We are going to describe below in a more precise way an example of aprocess of preparation of a tungsten catalyst supported by wetimpregnation at a controlled pH.

Preparation of the support: The raw support is first of all washed. Thenit is dried followed by calcination at 500° C. The obtained solid iscrushed in a mortar; only the particles of diameters ranging from 80 to400 mm are conserved.

Impregnation: The precursor of the used tungsten is ammoniumparatungstate (NH₄)₁₀W₁₂O₄₁.5H₂O. The use of this salt allows to leadonly to the formation of WO₃ after calcination.

Two solutions of ammonium paratungstate 0.005 M, whose initial pH wereadjusted respectively to 4 and 2 were prepared by the addition of NH₄OHand 4M of HNO₃. 40 ml of each of the two solutions were put in contact,at room temperature and stirred for 5 min., with 8.00 g of TiO₂ preparedas explained below:

We were careful to keep the inital values of the pH conserved during thewhole duration of impregnation. After that period of impregnation of 3hours at 70° C., the excess solution was eliminated by evaporation in anoven at a temperature of about 100 to 110° C. for 12 hours. They werethen calcined for 15 to 16 hours at 500° C. The catalyst C4 is thenobtained from the second solution (pH=2) after reduction at 510° C. for40 hours.

One has to note that under those conditions, the concentration in metalof the solution of the metallic percursor varies a little during theimpregnation. One can note, as an indication, that the fixation of anatomic layer of WO₃ leads to a loss in tungsten in the ammoniumparatungstate solution of about 10%.

The best catalytic results have been obtained using catalysts preparedfollowing the methode described by “Ipatieff” (J. Am. Chem. Soc.70(1948)533), in which the excess solution has been eliminated byevaporation.

Of course, the molybdenum catalyst is obtained by the same scheme as theone used in the example of tungsten catalyst.

Catalytical Tests

In order to evaluate the capacities of the prepared catalysts, we haveperformed a series of catalytic tests using the catalytic reactorpresented in FIG. 1, which is equally used for the reduction of theoxides.

A hydrocarbon such as 2-methylpentane has been used in order todetermine the reduction temperature which is necessary in order toactivate and stabilise the catayst. The reaction temperature is alwaysat 350° C., except when the reduction takes place at lower temperature:In this case, the reaction temperature is the same as the reductiontemperature.

The added hydrocarbon to the hydrogen flux is injected in a refrigeratedtrap by the melted anisole(−37.5° C.), which enables to obtain a partialpressure of the hydrocarbon at about 666.6 Pa (5 torrs). The usedhydrogen flux is 0.03 l/min, in order to ensure a passage time of about6 minutes for the hydrocarbon over the catalyst.

At the reactor exit, the mixture of the hydrogen and the hydrocarbon isanalysed by in line gas chromatography. In this order, a capillarycolumn of 50 m length and a 0.53 mm interior diameter was employed. Thisis diluted by helium(0.00122 l/min, which corresponds to 0.0927 m/s).The stationary phase is dimethylpolysiloxane.

The temperature program applied to the colon is composed of a firstlevel of 20 min at 35° C., followed by a linear increase in thetemperature of 25° C./min, then a third level at 110° C. for 30 min.

The chromatograph used in this work is provided with a flame ionizationdetector, supplied by a mixture of air/hydrogen, and stabilized at 200°C., meanwhile the injector is maintained at 150° C.

The spectrum obtained by gas chromatography is analysed in order tocalculate the product distribution, the selectivity and the activity aswell as the rate of the reaction.

The study of the C1 and C2 catalysts obtained bv mechanical mixture(theamount of tungsten corresponds to a monolaver of WO₃ on TiO₂)

The study of the catalysts behaviours in which we have applied levels ofone hour at increasing temperatures, shows a significant activity at460° C. reduction temperature. As a result, these catalysts were studiedin function of reduction time at 460° C.

The diagrams present in FIG. 2 represent the most interesting resultsconcerning the catalyst before and after calcination for 12 hours at500° C.

FIG. 2 shows also the influence of the calcination step in thepreparation of the catalyst which is responsible for the lowering of theactivity for longer periods of reduction. However, both catalysts C1 andC2 present a very high selectivity of about 90%.

Study of the the suDported catalyst C3 (deposit of the equivalent of onemonolaver of WO₃), obtained by impregnation at a non-controlled nH.

As in the precedent cases, the catalyst has first been treated at levelsof one hour at increasing temperatures. The catalyst has thus beentested at temperatures ranging between 380° C. and 700° C. However, theinventors did not found values of conversion which were comparable tothose obtained by the catalyst C4 studied below.

For that matter it is suitable to note that fact to operate attemperatures as high as 700° C. can lead to the obtenion of TiO₂ in therutile state.

Study of the supDorted catalyst C4. obtained bv impregnation at acontrolled pH and bv evanoration of the excess solution.

Catalytical tests (FIG. 3) have also been realised under hydrogen flux,on 50 mg of catalyst, and a partial pressure pressure of 2-methylpentaneof 893.3 Pa (6.7 torr). The reaction temperature at the beginning was350° C. In a first step, the evolution of the surface in function of thetime of reduction up to 2400 minutes was studied. In FIG. 3, certainpoints of measurements correspond to temperatures of reduction differentfrom 460° C., that temperature being then reported on the diagram.

In a second step, the activity at successive levels, at decreasingtemperatures from 500 to 250° C. was measured.

Otherwise, the activity of the catalyst C4 was also tested for theisomerization of 4-methylpentene-1(see table 4).

When the metallic mass was increased to 22% of tungsten, which isequivalent to 5 atomic layers (catalyst 4), the observed values for thecoversion and selectivity were 8% and 95% respectively. At an equalmetallic mass, the impregnation of the support at an acidic pH enablesto increase the conversion.

Furthermore, it is possible to reach all the values of conversionbetween 0 and 75% by varying the temperature of reaction between 300 and500° C. ***This associates withy a decrease in the selectivity inisomerization: At very low rates of conversion, the isomerizationreaches 100%, whereas it is only 4.5% at 510° C. for a conversion of 75%(table 3).

Once the catalyst is prepared under hydrogen at a given temperature ofreduction, different products can be obtained for a given reactant bychanging only the temperature of the reaction.

EXAMPLE 1 Catalyst C4 (5 layers) Prepared After Reduction Under Hydrogenat 500° C. for 40 Hours Using 2-methylpentane.

The results are summarized in the following table:

TABLE 3 Evolution of the activity and the selectivity in function of thereaction temperature for the catalyst C4 (5 layers). T reaction° C.Activity % Selectivity in isomers % 350 8 95 330 3.6 97 300 1.2 100 3152.3 98 380 12 81 350 8 94 400 15 68 420 16 49 460 26 15 510 75 4.5

The temperatures indicated in table 3 are classified in chronologicalorder from the bottom, the first temperature(350° C.) correspondingtherefore to the first tested temperature of the reaction.

We can observe that returning to 350° C. (6th value) leads to the samevalues of activity in isomerization (respectively 8% and 94% versus 8%and 95%), found for the reaction temperature during the firstmeasurement: The catalytic system is therefore perfectly stable.

EXAMPLE 2 Catalyst C4 (WO₃, (5 layers)/TiO₂: Using 4-methylpentene-1Reactant

The results concerning the evolution of the activity and the selectivityin function of the reaction temperature for the catalyst C4 using4-methylpentene-1 reactant are summnarized in table 4.

TABLE 4 Evolution of the activity and selectivity in function of thereaction temperature using catalyst C4. T reaction ° C. Activity %Selectivity in isomers % 350 58 92 250 57 90

As we can see from the results given in the following tables 5 and 6,that comparable results were obtained by using the MoO₃/TiO₂.

TABLE 5 Conversions and selectivities of MoO₂, MoO₃, and MoO₃/TiO₂ forthe n-hexane reactant at different reaction temperatures. ¹⁾ =Commercial compounds prior to reduction; ²⁾ = The initial state in orderto obtain the catalyst by impregnation Temp. de Conversions %Selectivities % réaction MoO₃/ MoO₃/ ° C. MoO₂ ¹⁾ MoO₃ ¹⁾ TiO₂ ²⁾ MoO₂¹⁾ MoO₃ ¹⁾ TiO₂ ²⁾ 280 2.0 3.9 3.7 100 97.4 100 300 5.5 8.2 15.2 10094.3 98.8 320 15.9 16.1 24.1 94.4 86.4 82.9 340 24.3 20.2 37.1 82.0 84.875.1 360 41.7 34.7 64.7 69.1 77.4 57.1 380 50.0 47.4 69.9 67.1 61.5 43.2400 68.1 63.3 86.5 48.3 46.0 22.7

In the following table 6, the compounds C1 to C5 do not designate thectatlyst of the present invention but the products of the reaction (fivecarbon atoms C5 . . . ) of n-hexane. On the other hand, the otherabbreviations given in the table are: 22 DMP is 2,2-dimethylpentane,23DMP is 2,3-dimethylpentane, 2MP is 2-methylpentane, 3MP is3-methylpentane.

TABLE 6 Products distribution obtained by the reaction of n-hexane onMoO₃/TiO₂ at different reaction temperatures. Reaction temperatures, °C. Products 400 380 360 340 320 300 280 Cracking C1 26.2 18.1 13.5 8.76.1 0 0 C2 18.6 12.1 9.5 4 2.9 0 0 C3 19 14.7 10.7 6.1 3.6 1.3 0 C4 8.86.7 5.1 2.8 1.5 0 0 C5 4.7 5.3 4.7 3.3 3 0 0 Isomerization 22DMP 1 1.4 21.8 2.2 0 0 23DMP 2.3 4.3 5.5 6.3 6.1 6.3 7.3 2MP 11.1 22.3 30.3 41.747.3 59.7 64.4 3MP 7.7 14.9 19.7 25.4 27.7 34.8 36.4 2MP/3MP 1.5 1.5 1.51.6 1.7 1.7 1.8

The catalysts of the present invention are particularly useful in thefield of organic chemistry and in particular in petrochemistry.

The catalysts described in the present invention can intervene inisomeization, dehydrogenation, and/or hydrogenolysis reactions ofsaturated organic compounds, in particular of alkanes as well as in theisomerization, hydrogenation, dehydrogenation, and/or hydrogenolysisreactions of mono or poly-insaturated organic compounds, in particularalkenes and alkynes.

Of course, the invention is not limited to the embodiments described andpresented in the drawings given as enclosures. Modifications arepossible, especially from the point of view of the use of differentelements or by substitution of equivalent techniques, without beingoutside the field of protection of the invention.

What is claimed is:
 1. A polyvalent bifunctional catalyst, characterizedby the fact that said catalyst comprises, deposited on a support TiO₂,an oxide or a mixture of metallic oxides of MO₂ type prepared by thereduction of the corresponding MO₃ oxide(s), and wherein said polyvalentbifunctional catalyst has a metallic-acidic surface that can catalyze atthe same time redox reactions and acid-base reactions.
 2. A catalystcorresponding to claim 1, characterized by the fact that the metal(s)forming the oxide(s) are chosen from the group formed by W and Mo.
 3. Acatalyst corresponding to claim 1, characterized by the fact that themetallic oxide obtained by reduction on the support is WO₂.
 4. Acatalyst corresponding to claim 1, characterized by the fact that themetallic oxide obtained by reduction on the support is MoO₂.
 5. Acatalyst corresponding to claim 1, which is characterized by the factthat the metallic oxide(s) MO₃ which are reduced to MO₂ are deposited ona support TiO₂ which itself could be deposited on a substrate havinglarger surface area than TiO₂.
 6. A catalyst corresponding to claim 5,characterized by the fact that the said substrate is selected from thegroup consisting of SiO₂, Al₂O₃ and a zeolite.
 7. A catalystcorresponding to claim 1, characterized by the fact that the reductionprocess takes place under a gaseous flux containing at least hydrogen attemperatures between 380° C. and 550° C., during at least 6 hours, at aflow rate between 0.010 l/min and 0.050 l/min, preferably 0.030 l/min,with a volume between 90% and 100% of hydrogen.
 8. A catalystcorresponding to the claim 7, characterized by the fact that thereduction take place under a gaseous flux containing hydrogen and agaseous hydrocarbon compound which undergoes a chemical reaction usingthis catalyst.
 9. A catalyst corresponding to claim 8, characterized bythe fact that the gaseous hydrocarbon is present under a partialpressure range between 666.6 Pa and 1999.8 Pa.
 10. A catalystcorresponding to claim 1, characterized by the fact that the oxide(s)MO₃ are deposited in atomic layers on a support before being reduced toMO₂, this reduction process having no effect on the number of layers.11. A catalyst corresponding to claim 10, characterized by the fact thatthe number of atomic layers of MO₂ present on the surface of the supportranges between 1 to
 8. 12. A catalyst corresponding to claim 1,characterized by the fact that it contains in weight 5.4% and 27% ofMoO₃, which corresponds to 4.8% to 24% in weight of MoO₂.
 13. A catalystcorresponding to claim 1, characterized by the fact that it contains inweight between 6% and 30% of WO₃, which corresponds to 5.7% to 28% inweight of WO₂.
 14. A process for obtaining a catalyst according to claim1, comprising: preparing a mechanical mixture from one or many MO₃oxides with TiO₂ alone or TiO₂ deposited on a substrate having largersurface area than TiO₂, this mixture containing an amount of the metal Mwhich varies between 5% and 25%, crushing the mixture prepared in theprevious step, and reducing at 460° C. the oxide(s) MO₃, thus depositedas corresponding MO₂ oxides under a flux of a gas containing at leasthydrogen over the oxides MO₃.
 15. The process according to claim 14,characterized by the fact that the step of depositing the oxide(s) MO₃takes place by mechanically mixing the crushed MO₃ oxide(s) with TiO₂ orTiO₂ deposited on a substrate having larger specific surface area thanTiO₂.
 16. The process according to claim 1, comprising: washing thecrude support, followed by drying and calcination, crushing the obtainedsolid, then sieving it, depositing the MoO₃ oxide(s) on the TiO₂ supportor TiO₂ deposited on a substrate having larger surface area than TiO₂ byimpregnating the so called support with a solution metal M salt(s),calcinating the obtained product in order to form the MO₃ oxide(s), andreducing preferably at 510° C. the MoO₃ oxide(s) to the correspondingMO₂ oxides by passing a gaseous flux containing at least hydrogen overthe MoO₃ oxide(s).
 17. The process according to claim 16, characterizedby the fact that only the particles diameters vary between 80 μm and 400μm are kept following sieving.
 18. The process to claim 16,characterized by the fact that the impregnation of the metal M salt(s)takes place for 2 to 4 hours, preferably 3 hours, at temperaturesbetween 50° C. to 90° C.
 19. The process according to claim 16,characterized by the fact that in order to obtain WO₃ then WO₂ atungsten salt, is used.
 20. The process according to claim 16,characterized by the fact that in order to obtain MoO₃, MoO₂ amolybdenum salt, is used.
 21. The process according to claim 16, furthercomprising impregnating a support material at a constant pH which is inthe range between 1 and
 4. 22. The process according to claim 21,characterized by the fact that the metal salt solution is in excess withrespect to the support which is impregnated, the excess of the solutionbeing evaporated in an oven after impregnation at temperatures in therange between 80° C. and 120° C., for 10 to 14 hours.
 23. The processaccording to claim 14, characterized by the fact that the number ofatomic layers of MO₃ present on the surface of the support rangesbetween 1 to
 8. 24. The process according to claim 14, characterized bythe fact that the metal forming the oxides MO₂ are selected in the groupformed by W and Mo.
 25. A method of increasing the rate of a chemicalreaction, comprising: adding a catalyst corresponding to claim 1,characterized by the fact that this catalyst is added in a reaction ofisomerization, hydrogenation, dehydrogenation and/or hydrogenolysis odsaturated hydrocarbons.
 26. A method of increasing the rate of achemical reaction, comprising: adding a catalyst corresponding to claim1, characterized by the fact that this catalyst is added in a reactionof isomerization, dehydrogenation, hydrogenation and/or hudrogenolysisof mono or poly unsaturated hydrocarbons.